This invention relates to the basic oxygen steelmaking process. It relates particularly to the handling of the dust or fume collected from the hot effluent gases emitted during the basic oxygen steelmaking process.
In the basic oxygen steelmaking process (BOP), a tiltable, barrel shaped furnace is charged with controlled amounts of ferrous scrap and molten iron. The furnace is then turned upright and a water-cooled oxygen lance is lowered into the furnace to a predetermined position above the surface of the bath. Gaseous oxygen from the lance starts a number of chemical reactions in the furnace to melt the scrap and refine the iron into steel. Slag forming fluxes, such as lime, fluorspar and mill scale are added in controlled amounts to produce a molten slag which aids in the refining process.
The BOP produces large amounts of hot gases, primarily CO and CO.sub.2, and generates large quantities of dust or fume in the effluent given off from the furnace. The gases must be cooled and the dust and fume collected and removed from the effluent before the effluent is discharged to the atmosphere.
The dust is collected in large dust collectors such as precipitators and then stored in bins or silos until it can be disposed. The amount of dust produced by the BOP is considerable. In a typical BOP facility, almost 2 tons of dust are collected for every 100 tons of steel produced. Federal, state and local laws and regulations require that the large quantities of BOP dust must be stored, handled and disposed so that they are not a threat to the environment.
While BOP dust is primarily iron oxide, it often contains nonferrous metals, such as copper, zinc, lead, chromium, cadmium, etc., some of which are considered hazardous to the environment by the U.S. Environmental Protection Agency (EPA) if these nonferrous metals were permitted to leach into the environment and, therefore, require special handling and disposal. In order for the BOP dust to be considered non-hazardous by the EPA, the dust must not fail the EPA toxicity protocol (EPTOX) for eight potentially leachable heavy metals. Changes in the chemistry of the BOP dust and the alkalinity of the dust may affect such EPTOX test results. If the amount of each of the heavy metals measured in the leachate exceeds the limits set by the EPA, the dust would be considered by the EPA to be a hazardous waste that will require special handling and disposal. The nonferrous metals ("tramp metals") in the BOP dust are usually traceable to the scrap used in the steelmaking process. The amount of tramp metals in the dust is directly related to the quality of the scrap used. The best quality and most expensive scrap contains very few tramp metals. Poorer quality and less expensive scrap contains a higher Percentage of the tramp metals.
Steelmakers have been faced with a dilemma resulting from their desire to use a less expensive scrap in an effort to reduce costs. The use of the poorer quality and, therefore, less expensive scrap produces BOP dust which contains appreciable amounts of the tramp metals. Some of the BOP dusts containing these tramp metals may be hazardous when tested by EPA standards and, therefore, require more difficult and expensive forms of storage, handling and disposal of the BOP dust.